1. Field of the Invention
The present invention relates to a power saving circuit for switching mode power supplies (SMPS), and more particularly to a power saving circuit of a power factor adjusting type, which can reduce a power consumption of a primary winding in a transformer in display power management.
2. Discussion of Related Art
Being a circuit mainly for saving energy, in general, the SMPS power saving circuit should comply with the EPA(Environmental Protection Agency) standard of the U.S.A. or the NUTEK (narings och teknikutveckling verket) of the Sweden to obtain a product approval, at which the product can be mark of the approval.
______________________________________ EPA NUTEK ______________________________________ DPM on mode or Standby mode below 15W below 15W DPM off mode below 5Wow 8W ______________________________________
Where the power consumptions are values obtained in tests at an AC voltage of 230V max., which power consumption standards should be satisfied.
FIG. 1 illustrates a system showing a related art boost up power factor adjusting circuit using a power feed back.
Referring to FIG. 1, the related art boost up power factor adjusting circuit is provided with a power source 1 for rectifying an AC power, a power factor adjusting unit 2 for adjusting a power factor of a voltage rectified through the power supply part 1, an SMPS 3 for switching the voltage provided through the power factor adjusting unit 2 to a peripheral circuit, and a microcomputer 4 for controlling the SMPS 3 according to a DPM. The unexplained reference symbol 2a denotes a power factor adjusting part, L1 denotes a choke coil D1 denotes a diode, C1 denotes a condenser, and Q1 denotes a field effect transistor.
In the related art power factor adjusting circuit when an external AC power is applied, the power supply part 1 rectifies the AC power and provides to the power factor adjusting unit 2. The rectified DC voltage is then removed of an AC component through the choke coil L1, smoothened through the diode D1 and condenser C1, provided to the SMPS 3, and converted into a required voltage for a peripheral circuit. In this instance, the power factor adjusting part 2a in the power factor adjusting unit 2 controls a switching operation of the field effect transistor Q1 according to an AC voltage for adjusting a power factor of the applied AC power. That is, a power factor of the AC voltage is improved such that a current wave of the AC voltage comes closer to a sinusoidal wave, for smooth supply of a DC voltage rectified through the diode D1 and the condenser C1 to the SMPS 3. In the meantime, the microcomputer 4 controls the SMPS 3 such that a high signal is provided to the SMPS 3 in a DPM on mode or standby mode, to cut off an output voltage from the SMPS 3, for reducing a power consumption, and a low signal is provided to the SMPS 3 in a DPM mode for providing a regular voltage.
However, since the related art power factor control circuit only controls the output voltage from the SMPS 3 by the microcomputer 4 regardless of the control of the field effect transistor Q1 by the power factor adjusting unit 2 in the DPM mode, which causes to consume a power at an input terminal on the SMPS 3, the related art power factor control circuit has a problem in that the DPM standard can not be complied with.